A TEST OF GENERAL SHEAR VERSUS FOLDING FOR THE ORIGIN OF A CRYPTIC STRUCTURAL FEATURE, NASON TERRANE, WASHINGTON

The Nason terrane of the Cascades Crystalline Core is a complex tectonostratigraphic terrane which has been the topic of much study due to its deformational history and importance in the debate surrounding the Baja B.C. hypothesis. Structural patterns along Nason Ridge in the central part of the terrane have been interpreted as the result of either the presence of a major shear zone (Magloughlin, 1990) or the development of fold interference patterns (Miller et al., 2006). Distinguishing fold-controlled lineations from shear zone produced lineations can be a structurally complex, but important problem. This study aims to provide a better understanding of high-temperature deformation which may be obscured due to extensive recrystallization.

Previous observations revealed a structural zone ~1 km wide striking roughly NW-SE. Fold hinges and mineral lineations approach subhorizontal orientations with increased distance from the center of the zone, indicating that this zone has a fundamentally different structural fabric than the surrounding terrane. In the zone, mineral lineations cluster tightly, plunging steeply NE. Fold hinge lines are more dispersed along the trend of the zone, possibly reflecting incomplete rotation of pre-existing fold hinges.

AMS analysis done at the Institute for Rock Magnetism at the University of Minnesota indicate the magnetic fabric correlates well with lineations and foliations measured in the field, and likewise shallows to near-horizontal away from the center of the zone. Quantification of the strength of the AMS fabric reveals an oblate spheroid which is interpreted as a proxy to an oblate strain ellipsoid, indicating a component of pure shear. Asymmetrical microstructural features include tailed porphyroclasts, mineral fish, and domino clasts. Sense-of-shear indicators also agree with the hypothesized NW side up.

It is clear from the agreement between outcrop scale structures, magnetic fabric, and microstructures that a significant structural perturbation is tightly centered on the zone. The highly focused nature of the zone and the lack of structures created by fold interference indicates that superposed folding is unlikely. These data, coupled with geothermobarometric constraints, point to a zone of displacement best characterized by general shear.